SLI-LABAT · Stagnation Layers in Laser Ablation Based Analytical Techniques

Stagnation Layers in Laser Ablation Based Analytical Techniques

I. Motivation. Laser ablation, leading to heated and partially ionized plasma plumes, underpins a number of key commercial analytical techniques employed across a wide range of domains including materials science, biopharmaceuticals, security/forensics, environmental monitoring, etc. The overarching goal of this proposal is to significantly increase the potential performance of laser ablation analytical techniques by substituting the traditional single ablated plasma plume by a stagnation layer of controllable density, temperature, geometry, composition, etc. formed at the collision front between two (or more) counter- propagating plumes. For example it is known that complex and expensive femtosecond laser plasma systems tend to preferentially generate nanoparticles and so it is no surprise that this approach is now already employed in commercial LA-ICP-MS. However, there is already clear evidence from this project that nanoparticles are preferentially formed by colliding (nanosecond) laser produced plasmas in gases. Hence colliding plasmas, formed by a much less complex and cheaper Q-switched laser, in an ambient gas atmosphere could lead to a significant simplification of the laser ablation step over the femtosecond laser driver case and a concomitant reduction in cost. II. Main results. A bespoke experimental system to form and diagnose colliding plasmas from a wide range of geometries has been designed, constructed and tested (Figure 1. in the attached pdf file). We have observed and tracked the formation of stagnation layers at the collision front between colliding plasmas of the same (homogeneous) and different (heterogeneous) metallic plumes (Figure 2. in the attached pdf file). We have observed and analysed the controlled formation of metallic nanoparticles by the stagnation layer detected by deposition onto silicon substrates. We have driven, observed and analysed the controlled formation of complex multi-element nanocomposites (e.g., Cu-TiO2 and C-Ti) in heterogeneous colliding plasma experiments (Figure 3. in the attached pdf file). III. Conclusions In comparison to single plume deposition the stagnation layer persists for a significantly longer time period than in the case of a single plume. As a result, debris (e.g., large chunks of target material) is significantly reduced leaving almost exclusively nanoparticles, atoms and ions which is a very important advantage for improving the limit of detection in laser ablation analytical sciences by having improved the atomisation step. For the heterogeneous nanocomposites we have strong evidence (almost published) that we can control the stoichiometry better than in single plume ablation. IV. Impact (Economic and/or Social) Cu-TiO2 nanoparticles play a key role in cancer diagnosis and treatment. What is crucial to the efficacy of these techniques is the stoichiometry of these nanocomposites. Hence increasing the degree of control over them is a significant step forward on this area of biomaterials physics. As alluded to above, the formation of nanoparticles in the aerosol step for many analytical techniques such as ICP-MS has improved greatly the limit-of-detection but at the cost of complex and expensive ultrafast laser technology. These techniques are essential in manufacturing quality control, security, forensics, environmental monitoring, diagnostics and drug discovery. We have shown that colliding plasmas can produce such an effect but with simple and relatively inexpensive Q-switched laser technology with the potential to reduce the unit cost for such techniques very significantly.

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